Debug --diff --diff-methods creates false-positives

[muglug]

On Psalm latest

./psalm --clear-cache
git checkout b3f6b56
./psalm --threads=8 --diff --diff-methods --show-info=false
./psalm --threads=8 --diff --diff-methods --show-info=false
git checkout 6f6e265
./psalm --threads=8 --diff --diff-methods --show-info=false

Expected: No issues
Actual: A whole bunch of PossiblyUnusedMethod issues that would normally be covered by the PHPUnit plugin

This is probably a bug in Psalm, rather than the plugin, but storing here because it affects the plugin.

[ktomk]

I've seen things not working well (e.g. missing negatives) when not invoking with --no-cache when changing php versions / configuration / setup / switching revisions and all the like. Just saying as from my current understanding each psalm invocation should be with --no-cache to eliminate (unwanted) side-effects, especially with bug/issue reporting.